1. Field of the Invention
The present invention relates to an apparatus and method for thinning predetermined frequency components in a signal in order to compress information of the signal to be transmitted or recorded.
2. Related Background Art
In recent years, there is the increasing delivery of information such as music by a cable broadcast, wireless broadcast and communication technique.
Sometimes, lines used in broadcast or communication do not have bandpass characteristics wide enough to transmit audio signals up to its high-frequency part without attenuation. The technique used in such a case is in that the device sending an audio signal boosts the high-frequency part of the audio signal to be sent (that is, amplifies the high-frequency part with a larger amplification factor than the other parts), then sending out the audio signal having the boosted high-frequency part.
The boosting of a high-frequency part has been achieved by passing an audio signal to be sent through an equalizer shown in FIG. 8 and then amplifying it, for example.
The equalizer of FIG. 8 comprises a capacitor C connected between an input and output terminals, a resistor R1 also connected between the input and output terminals and a resistor R2 having one end connected to the output terminal and another end connected to ground. Thus, a signal input to the input terminal is attenuated with larger attenuation factor in the bands except the high-frequency part than in the high-frequency part, and then output from the output terminal.
However, when a signal (for example, a signal having a removed high-frequency part) having a high-frequency part steeply attenuated above a given frequency, such as an audio signal composed of compressed information, is boosted in the high-frequency part and then transmitted, there have been problems in the receiving side that it is difficult to exactly restore the received signal to its original signal (the signal before boosting). If an audio signal is to be transmitted, this problem may result in degrading the quality of the reproduced audio.
The invention has been made by considering the above situation. It is an objective of the present invention to provide an apparatus and method for thinning frequency components of a signal, allowing the compression of the bandwidth of the signal in such a manner that even the signal having a high-frequency part steeply attenuated may be exactly restored.
It is another objective of the invention to provide an apparatus and method for thinning frequency components, allowing the compression of the information of an audio signal while keeping high audio quality.